Reverse osmosis (RO) water is not the same as distilled water, although both methods effectively purify water by removing contaminants and dissolved solids. While both processes aim to produce water nearly free of impurities, their underlying scientific mechanisms are fundamentally different. Distillation uses a phase change to separate water from contaminants, while RO uses mechanical pressure and a specialized membrane.
The Distillation Process
Distillation relies on evaporation and condensation to separate water molecules from non-volatile substances. The process begins by heating the source water to its boiling point, typically 100°C (212°F), converting the liquid into steam. This phase change leaves behind most impurities, such as minerals, salts, and heavy metals, which cannot vaporize at that temperature.
The resulting steam is directed into a cooling chamber. As the steam cools, it reverts back into purified liquid water, which is collected in a storage container. This method is effective because virtually all dissolved solids and inorganic compounds have significantly higher boiling points than water and are left behind. The high temperature also inactivates bacteria and viruses.
The Reverse Osmosis Process
Reverse osmosis (RO) is a mechanical purification method that forces water through a semi-permeable membrane under high pressure. This pressure overcomes natural osmotic pressure, pushing water molecules through extremely fine membrane pores.
The RO membrane acts as a barrier, rejecting particles larger than approximately 0.0001 micron. This includes most dissolved salts, colloids, bacteria, and viruses. The removal mechanism relies on size exclusion and the electrical charge of contaminants, rejecting up to 99% of dissolved salts. The purified water (permeate) is collected, while the concentrated stream of rejected contaminants is flushed away as brine waste.
Fundamental Differences in Operation
The operational difference lies in the physical mechanism: phase change versus membrane filtration. Distillation is energy-intensive because it requires a large input of heat to boil the water and then cooling for condensation. This heating and cooling cycle makes distillation a slow process, often producing purified water at a low flow rate, such as one liter per hour in home units.
Reverse osmosis is a pressure-driven process that consumes less energy since it does not require heating the water to its boiling point. RO systems produce purified water more quickly and continuously, making them practical for on-demand household use. A drawback of RO is the production of wastewater (brine), which flushes rejected contaminants down the drain, while distillation leaves the residue in the boiling chamber.
Specific Contaminant Removal Comparison
The distinct mechanisms lead to differences in contaminant removal effectiveness. Distillation is effective against non-volatile impurities like heavy metals, salts, and minerals, as they are left behind when water vaporizes. However, distillation is less effective at removing volatile organic compounds (VOCs), such as benzene, which have boiling points near or lower than water. These chemicals can vaporize and re-condense, potentially re-contaminating the water.
RO is excellent at removing dissolved solids and most inorganic contaminants. However, it has limitations regarding certain low-molecular-weight substances and dissolved gases. Small, uncharged VOCs can sometimes pass through the membrane, often necessitating an additional activated carbon filter stage in home RO systems. Both processes remove naturally occurring minerals like calcium and magnesium, which can result in the water having a neutral taste.
Practical Uses for Each Water Type
The extreme purity of distilled water makes it the preferred choice for applications where the complete absence of dissolved solids is mandatory to prevent mineral buildup or interference. This includes use in medical devices, such as Continuous Positive Airway Pressure (CPAP) machines and humidifiers, as well as in laboratory testing and automotive cooling systems. Distilled water is also commonly used in steam irons to prevent scale formation on heating elements.
Reverse osmosis water is primarily utilized for general household consumption, including drinking and cooking, due to its convenience and continuous supply. Because RO water can retain trace amounts of minerals or can be passed through a re-mineralization filter, it often has a more palatable taste than completely demineralized distilled water. RO systems are also commonly used in aquariums, where specific water quality control is necessary, and in commercial beverage production.